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Sunday, October 12, 2014

Forecasting the 2014 Ebola Outbreak; Update #2

April 29, 2015My latest update regarding the history of the Ebola epidemic can be found here:http://dougrobbins.blogspot.com/2015/04/ro-and-history-of-ebola-epidemic-in.html-------December 17, 2014: The exponential growth rate of the Ebola epidemic continued for about two months after my original blog post in August, 2014. The exponential trend was broken in mid-October, 2014, thanks to global relief efforts and effective public health programs in the affected countries. Updates to my original charts can be found here:

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There is no clear end-point to the Ebola outbreak in West
Africa. The Ebola outbreak in West Africa, now involves about 9000 reported cases, with the likelihood of an equal number of unreported cases (10/17 update). Like ripples from a stone thrown into the water, the epidemic has the potential to infect heavily populated areas of west Africa, to spread across porous borders by people fleeing the epidemic, and to jump from large cities to other large cities. The following figure shows a timeline extrapolated from the exponential growth of the disease, and marked by points representing populations at risk. Africa is impoverished and lacks the resources to maintain a stringent defense against the epidemic. There are no firebreaks, and the world will be able to do very little to stop a general pandemic if the case numbers continue to grow well into the coming year.

Reports from the World Health
Organization (WHO) are increasingly bleak.
The WHO October 8th situation report states:

” …the situation in Guinea, Liberia, and Sierra Leone
continues to deteriorate, with widespread and persistent transmission of EVD
[Ebola Virus Disease]….the reported fall in the number of new cases in Liberia
over the past three weeks is unlikely to be genuine. Rather, it reflects a deterioration in the
ability of overwhelmed responders to record accurate epidemiological
data….There is no evidence that the EVD epidemic in West Africa is being
brought under control….”

The international community is increasing support to the
affected countries. New teams and
equipment are being sent from the United States, Great Britain, Cuba and
Nigeria, among others. However, the
scale and speed of the intervention appears too little and too late. At this point, we have to ask where this
epidemic is going, and when will it get there.

This is the third post on the topic of Ebola on this blog. The first two are here:

We can consider the future course of the epidemic by linking
our charts of case numbers to the geographical distribution of actual
populations. Here is a map of Africa
showing population density.

The colored outlines show populations which might be at risk
if the epidemic spreads across Africa. For illustrative purposes, these populations
are linked to the extrapolation of the cumulative number of cases, using a
contagion model with Ro = 1.63. In this
model, each case of Ebola infects (on average) 1.63 new victims after a
transmission and incubation period of 16 days.
This model fits the reported exponential growth of the disease to date.

A number of cases can be expected in developed countries as
a result of infected individuals traveling to new countries. The United States and Spain have already seen
such cases and subsequent secondary infections.
However, rich countries with well-developed medical systems should be
able to easily quell the epidemic, given only a few points of infection.

The Ebola epidemic is much more likely to spread through the
impoverished nations of Sub-Saharan Africa.
By the numbers, at the established rate of growth would spread the
epidemic to every individual in Guinea, Sierra Leone, and Liberia, totaling 21
million people, by the mid-year 2015. The
epidemic could spread to every individual in West Africa, 412 million people,
by autumn 2015. By year-end 2015, the
epidemic could spread to nearly a billion people in Sub-Saharan Africa by
year-end 2015, showing the astonishing power of exponential growth.

Of course, the epidemic will not unfold as a neat
mathematical formula. There are factors
which will impede the growth of the disease.
And there are factors which may accelerate the spread of the disease. As the rate of infection rises in a population, several
things may happen. Some of these things
will reduce the transmission of disease, and some things will increase the transmission
of disease. In affected populations,
intense public health campaigns are being conducted, which should reduce risky
behaviors and reduce the rate of infection.
Also, as the infected population approaches saturation, the rate of new
infections should decline. On the other
hand, high infection rates will strain the fabric of society, overwhelm
treatment facilities, and reduce the number of qualified professionals working to
contain the epidemic. And perhaps worst
for the rest of the world, at some point large numbers of Ebola refugees will
flee infected countries and carry the disease to new places.

Liberia
Case Count

Recent data reported by the World Health Organization seem
to show a slower rate of growth than in the earlier history of the epidemic. However, there is serious doubt about the
accuracy of recent reports, particularly from Liberia. The October 8th report from WHO
bluntly states “Evidence obtained from responders and laboratory staff in the
country indicates beyond doubt that there is widespread under-reporting of new
cases, and that the situation in Liberia, and in Monrovia in particular,
continues to deteriorate from week to week.”

The number of new cases appears to be declining; but this is
unlikely to be true considering other evidence.

Since early September, reports from the Liberian Ministry of
Health have been late, contradictory, and inconsistent. The most recent reports from Monrovia and
Freetown still indicate that all isolation units and hospitals are full, and
that patients are turned away to transmit the disease at home or in the
streets. Although we can hope that
public health measures are reducing the rate of transmission, we are in a state
of uncertainty, without reliable numbers to support that conclusion.

Adequate medical facilities are necessary to measuring the development
of the epidemic. If families of Ebola
patients believe that they will receive treatment, patients will be brought to
medical facilities, and data gathered about the progress of the disease. If patients are consistently turned away from
treatment facilities, there is no point to bringing stricken people to the
clinics. For the past two months, the
number of beds in Ebola clinics in Liberia has been only about 20 % to 25 % of
the number needed. Some 75% to 80% of
Ebola patients have been turned away, or left to suffer in the streets. In this environment, it is understandable
that families do not seek official care, but instead choose to care for loved
ones at home. This means that official
figures no longer have any validity in measuring the epidemic, and virtually
guarantees the spread of the disease.

Ebola
Clinic Beds

Specialized treatment centers are necessary for Ebola
patients. Without proper equipment and
training, medical personnel are at tremendous risk of contracting the
disease. [As of October 8th,
416 healthcare workers have contracted the disease, and 233 have died.] But throughout the epidemic, the number of
beds in proper treatment centers has lagged far behind the need. According to WHO, throughout the months of
September and October, the number of beds available to patients has been about
25% of the number needed.

In the 8th October report, WHO indicates that
there are about 1100 beds available to Ebola patients in qualified treatment
centers. The current need is for over
4300 patients, a shortfall of 3200 beds.
There is adequate care for only 25% of the patients.

The United States and Britain have mobilized military
resources to fight the epidemic. The United
States will build and staff treatment centers containing 1700 beds in Liberia, while
the British army will build and staff treatment centers containing 700 beds in
Sierra Leone. Both programs will require
about 60 days for implementation. This effort will still leave a shortfall of 800 beds, according to current needs.
It is likely that other organizations will also contribute new beds, and
perhaps bring the total number of beds to 4300 by the end of November.

The problem is the human tendency for linear thinking,
whereas problems in nature are generally exponential. By my estimates, the Ebola epidemic is
doubling about every twenty-nine days. (The
CDC calculates the epidemic is doubling every 20 days, by assuming a large
number of unreported cases). In 60 days, the epidemic will have grown four-fold. In
mid-September, when the US Army intervention was announced, about 1800 new beds
were needed, for a total of 2425 beds. If
the epidemic continues to grow exponentially, by mid-November about 10,000 beds
will be needed.

Death Rate

In my first post, I noted that diseases evolve quickly, and
that evolution causes the disease to become more contagious, and less
deadly. Living patients propagate the
virus better than dead patients. As
expected, the mortality rate for Ebola patients appears to be falling, from
about 70 percent to about 55 percent.

The chart shows cumulative
reported deaths as a fraction of cumulative reported cases, with a lag of eight
days. I would like to calculate a daily
death rate (an instantaneous function), but the official figures are reported
too sporadically for a meaningful chart.

Future
Course of the Epidemic

The equation used by epidemiologists to forecast an epidemic
looks like this:

Cumulative Cases= ((Ro/1+d)t)t

The term “d” is a decay parameter, which
reduces the rate of transmission over time.This could represent effective medical intervention, or a public
awareness campaign, or disease saturation within the population at risk.We might hope that the declining number of
reported new cases in Liberia reflects a real decline in new infections, but all
other evidence indicates that the people of Liberia increasingly distrust the
official treatment centers, and no longer bring patients to be counted.

We might also consider a parameter which
increases the rate of transmission over time.
Such a parameter might represent breakdown of social order, growing distrust
of medical facilities, or dispersion of the population fleeing the epidemic. In any event, the data quantifying the
epidemic are now uncertain, and there is little basis for assigning values to
parameters which may reduce or increase the rate of transmission.

While we must make every effort to stop Ebola within
the current area of active transmission, we should also realistically consider
what will happen if those efforts fail.
It is likely that neighboring nations will receive refugees through
porous borders, and some of those refugees will carry the virus. Ebola can be expected to spread along the
West African coast through coastal communities, and jump from infected large
cities to other large cities. Like
ripples from a stone thrown in the water, Ebola has the potential to expand
through heavily populated West Africa, and ultimately affect the entire
continent. At some point, perhaps at
20,000 active cases, or 100,000 active cases, the disease will not be stoppable
with the procedures that ended previous epidemics. According to the rate of growth and the
distribution of African population, the 2nd half of 2015 may be a
very bad time for Africa.

Some people do survive Ebola, and they are
surviving in increasing numbers. This
shows that the human organism can mount an effective immune response to
Ebola. Medical science has developed
vaccines for other deadly viruses. It is
inevitable that an effective vaccine will ultimately be developed and produced
on a large scale. The question is how
soon the vaccine can be produced and brought to protect the people at risk.

In considering the impact of the Ebola
epidemic, I am strangely reminded of something a colleague said to me on the
morning of 9/11/2001. The second plane
had just struck the World Trade Center.
Being quicker and brighter than me, she turned and said, “You realize, nothing will
ever be the same again”. Of course, I had not realized the impact of the event, and didn’t fully understand
until several years later. In the
same way, something with global impact has happened in West Africa. The world does not yet realize that everything has changed, and nothing will ever be the same again.

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* Cumulative Cases= ((Ro/1+d)t)t

I used a simpler exponential function for my
first model, and a simple rolling formula in Excel to create my contagion model
with Ro. In the equation shown, one “t”
must represent the time between initial and subsequent infection, and the
second “t” must indicate the life of the epidemic. I will update this post after I have some
time to play with the equation.

Chart Updates to my original post, using the original extrapolations. In the October 29 report, WHO presents revised figures that add about 3700 cases to the previous total. These cases were recognized through study of patient databases, and occurred throughout the epidemic period, and not only since October 22. The additional cases return the cumulative case number to my original exponential extrapolation, first presented on August 26th.It is uncertain whether the apparent flattening of the cumulative cases, observed through the month of October, is real or the result of under-reporting. Case reporting is increasingly late, and WHO cites data missing for a number of dates. I have now seen two anecdotal reports that give a more optimistic appraisal of the situation in Monrovia, indicating fewer patients are reporting to Ebola clinics, and fewer bodies are being collected from the city outside the clinics. Authorities disagree on whether the drop in patients shows a real decline in the epidemic, or avoidance of the clinics.

Personal report by Drew Hinshaw, WSJ, on awful situation in Monrovia. Patients now told to stay at home; families
told to leave patients alone. Increasing
numbers of patients on street, as compared to several weeks ago. Mobs of people moving on streets; people
trying to leave.

Infographic showing the rate of increase in cases over two months, and
the number of beds available for treatment.
Also shows the US commitment to build additional beds over the next two
months, which will provide about 1.5% of the needed beds in November.

4 comments:

Hi Doug, I have some concerns about Ebola cases in the US. I wonder about missed Ebola cases because they do not fit the "testing criteria". I had a patient recently (less than 10 days ago) that fit the bill EXCEPT she had not travelled from West Africa nor been around anyone from West Africa. She had travelled over the Christmas holidays to Dallas and had spent time in a local hospital (as a family member visiting another). She was sick 2 days before I took care of her, and she died the next day. I have practiced medicine for over 25 years. This patient "screamed" viral hemorrhagic disease. Now here is the kicker. I called the CDC (I was referred to my state health department). I called the Dallas Health Department (left a message - and no return call), I called the regional health department (I was told not to worry if the patient was not from West Africa). I called every possible resource available and GOT NOWHERE. The patient to my knowledge did not get tested. It has been less than 12 weeks since Texas had an active EBOLA case, and that person/nurse travelled. There were also thousands of health personnel and community exposures during that time. I can't quite convince myself that EBOLA cases are not possible outside of West Africa travel at this point. This also sends me back to your original plot graphs (and revised plot graphs). What if those graphs are still valid? Thanks for letting me write about this. I find that this topic is not regarded well with most people. Either it is terrifying to them and they begin to panic, or they look at me like my theories are ridiculous. I guess I need to be glad I was ignored by the agencies. I don't think our state or country is ready for another possible outbreak.

Thanks for writing. I apologize for not noticing your comment earlier!! I agree with your concern. We need to recognize and know this disease wherever it occurs. If it is possible for one case to be ignored, it is possible for dozens of cases to be ignored, and then the exponential growth may be difficult or impossible to stop. And if this case of suspected hemorrhagic disease is not EBOLA, what is it? Is it Dengue Fever, communicable by mosquitoes? I lived in Houston for 20 years, and south Louisiana for another 5 years. We have lots of mosquitoes!I continue to worry about a stealth epidemic, particularly in Africa. The epidemic may be quelled in the areas with vigorous intervention, but I think it is possible that it is now spreading beyond the borders of Guinea. Vigilant medical professionals and knowledgeable citizens are our best protection.

more,that is still you may do something to expand on it.yhanks for sharing with us.I think I am going the clearness in your post is simple spectacular and I can take for granted you are an expert on the field.Well with your permission.Like your way of seeing this things. i'll be checking in on a regularly now....keep up the good work.

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